TinyMCE: Improve styling for the Classic Block

[azaozz]

Currently we use wp-includes/css/editor.css to override some of the styling for the TinyMCE UI to make it consistent with the styling on the (old) Edit Post screen, and to style the "Advanced Link" modal.

Then, as the styling of the Block Editor is considerably different, we override the overrides to style the Classic Block toolbars. That is a left-over from the Gutenberg plugin where it needed to override core's styles and brings some visual bugs/inconsistencies. Now we can change editor.css to apply styles only where needed.

[azaozz]

As far as I see best would be to split editor.css into several parts:

• Styles for the "Advanced Link" modal.
• Overrides for TinyMCE's skin.min.css for the editor buttons icons (we use Dashicons for most).
• Overrides for the Classic Editor (for the old Edit Post screen and any plugins that use wp_editor()).
• Overrides for the Classic Block. These need to be added/improved here: ​https://github.com/WordPress/gutenberg/blob/master/packages/block-library/src/classic/editor.scss.

[azaozz]

In 46173.patch:

• First run :)
• Split editor.css into classic-editor.css, wplink.css and tinymce-icons-override.css.
• Add an editor-buttons "group" to script-loader for back-compat.
• Add a param to wp_enqueue_editor() and _WP_Editors::enqueue_default_editor() to distinguish which editor is being loaded, 'classic" or 'block'.

This is the first half of the patch. The second needs to go in the ​classic block scss, and both have to be added at the same time to core to avoid breaking the classic block's styling.

[audrasjb]

As per today's bug scrub, we are going to move this one to the next Release since beta 3 and RC are approaching.
Adding early keyword too, since this one will need some coordination :-)

[azaozz]

Looking at this again, it's a good idea but maybe should be done once the old Edit Post screen in fully deprecated. Then we can clearly separate the old editor styling as it will be only for plugins and the Classic block.

Closing as maybelater for now. Feel free to reopen with other ideas (and a patch?).